Change the way we cache events

Add get_presence_list_accepted to the broken caches in synchrotron

Remove event fetching from DB threads

Fix email notif From

Add a separate process that can handle /sync requests

Change def of small delta in presence stream. Add metrics.

Move insert_client_ip to a separate class

brand the email from header

Change CacheMetrics to be quicker

Poke notifier on next reactor tick

Make get_joined_hosts_for_room use get_users_in_room

Add methods to events, account data and receipt slaves

Load push rules in storage layer so that they get cached

We change it so that each cache has an individual CacheMetric, instead
of having one global CacheMetric. This means that when a cache tries to
increment a counter it does not need to go through so many indirections.